1. Field of the Invention
This invention deals with the evaluation of mineral ores and the identification of components in the ores and the ability to selectively determine with a high degree of accuracy the content of such ores. In specific, by using the apparatus and method of the present invention, wanted or unwanted chemical species can be identified, thus making this an effective screening tool for rapid analysis of ores in mining applications. It is also anticipated that the present invention may be deployed in any application for which the identity of a chemical species is sought.
2. Description of Related Art
The need for instantaneous evaluation of mineral contents on moving belt systems has led to the necessity of remote sensing techniques to instantly differentiate between corresponding minerals to select desired mineral ores from those that are undesirable. This selection may be based on selective influence of the different kinds of irradiation on those minerals, whereby discrete pebbles are analyzed within a fraction of a second for a characteristic that can distinguish between xe2x80x9cacceptxe2x80x9d and xe2x80x9crejectxe2x80x9d fractions. At present when dealing with moving belt systems, mineral evaluation is mainly performed using photometric, radiometric, steady-state luminescence, gamma-ray or X-ray fluorescence, and neutron absorption characteristics are mainly used for mineral evaluation.
U.S. Pat. No. 4,365,153 to Siegel describes the detection of certain minerals, including zinc, tungsten, fluorine, molybdenum, mercury, and other metals using a photoluminescence method. The basis of selective detection is that the lifetimes of photoluminescent emissions of many industrial minerals are much longer than the lifetimes of photoluminescent emissions of background materials that are likely to occur at the Earth""s surface. The measurement of the photoluminescent response measured here is based on a response-retention factor and not on a direct readout at the time of irradiation. The detection here is based on taking a delayed readout to identify the minerals of economic interest after enough time has elapsed to enable unwanted sources of photoluminescence to decay.
Broicher (U.S. Pat. No. 5,410,154) describes detection of quality alterations contained in goods as they progress on a moving belt. The use of laser-induced luminescence, reflected light, and temperatures including detection of signal intensity and the fading behavior of the signals in definite spectral ranges is disclosed. This approach was also used by AIS Sommer GmbH of Germany, in their laser-induced fluorescence (LIF) analyzer for quality control in minerals and mineral processing. The LIF analyzer includes two light detector systems with three photomultipliers each, which evaluate three spectral bands in two time windows each. Such a system was employed in the Kirumna phorphorus iron ore mine in Sweden.
The limitation of LIF analysis is that its accuracy depends on the complexity of the composition of the ore and the concentration and fluorescence properties of the critical minerals in relation to all the other minerals present.
Another approach to differentiation of samples is described by White (U.S. Pat. No. 4,423,814). Here the natural luminescence of the accept and reject fractions is similar. The basis of separation of magnesium-bearing ore particles is obtained by first conditioning the exposed magnesium-rich mineral with a surface coupling agent of hydroxquinoline, then irradiating the conditioned ore to excite and induce fluorescence and then effecting separation of the magnesium-rich minerals from the lean ore particles by detection of the difference of the intensity of the fluorescence. This method detects ores having high magnesium contents on the surfaces of the pebbles.
Many similar methods have been used in the past to screen samples based on physico-chemical properties. Another example of this is selective flotation of phosphates and dolomites. As stated above, the difficulties of selective flotation of phosphates with high magnesium content are explained by similar physico-chemical properties of phosphates and dolomites. This problem exists also with other ores, thus making it desirable within the mining field to find separation systems that can quickly distinguish between wanted and unwanted ore fractions.
Alexander (U.S. Pat. No. 5,847,825) describes a method and apparatus for detection and concentration measurement of trace metals using laser-induced breakdown spectroscopy (LIBS). LIBS is a simple, rapid, real-time analytical technique based on the analysis of the spectral emission from laser-induced sparks or plasma. Pulsed laser radiation is first focused to a small spot on the sample material. When power densities exceed hundreds of MW/cm2, a high-temperature, high-electron-density laser microplasma is formed. The temperature of this plasma initially is very hot: 104 to 107 xe2x96xa1 C. At such a high temperature, any sample material is broken down, vaporized and ionized. As the plasma cools down to the point when neutral atoms in excited states are formed, the excited species relax and emit optical energy at characteristic wavelengths. The emission can then be spectrally resolved to identify the elemental species that are present in the sample based on the presence of the characteristic lines.
It has been shown that LIBS is practical in situations, which require very fast, real-time measurements with no sample preparation. One such method is a penetrometer system for subsurface spectral analyses; such a system being described in Theriaul et al. (U.S. Pat. No. 6,147,754). Here, elemental identification using a cone penetrometer unit is achieved by using atomic spectral analyses of contaminants that are stimulated by a laser-induced breakdown of the soil containments to be determined.
Potzchke (U.S. Pat. No. 5,042,947) describes a scrap detection system using another LIBS system. Here a method and apparatus is disclosed whereby metal particles are detected in the context of being contained in compositions of alloying metals. Sorting of metal particles is effected by a plasma process, which cleans and then identifies the particles of interest. However, these metal particles are not in ion form as are those contained in mineral samples.
It is therefore an object of the invention to provide a LIBS system that is capable of detection of trace metals in an ore sample, such metals even being in an ionic species form.
It is a further object of the invention to provide a low-cost multi-element analysis system adapted to being compact and requiring no sample preparation.
It is another object of the invention to provide a system for sorting mineral ore samples which allows for rapid sorting of minerals in a moving belt system.
The present invention comprises a system and a method for mineral sorting and detecting, including remote sensing, and more particularly, for real-time detection and content evaluation of minerals or trace concentrations of elements in materials as they are conveyed on a moving belt. The instant invention employs a laser-induced breakdown spectroscopy (LIBS) system wherein intensity ratios of the emission lines characteristic for specific elements or minerals enable detection of the same while on a moving belt. Because associated minerals have different chemical compositions, namely, major or minor elements, the relative intensities, defined by their characteristic spectral lines, enables all phases to be consistently identified and assessed within a short time that is consistent with both LIBS and the moving belt system.
The steps to the invention include establishing a spectral signature ratio for at least one predetermined substance, the signature having a first intensity at a first wavelength relative to a second intensity at a second wavelength, applying pulsed laser energy to the mineral sample whereby plasma is produced, obtaining the spectral intensity of the plasma at the first and second wavelengths, and calculating the ratio of the spectral intensity at the first wavelength by the spectral intensity at the second wavelength whereby the presence or absence of the substance may be resolved.
It should be noted that the present invention permits the mineral sample to be analyzed substantially in real-time while in transit, such as moving on a conveyor belt. In a preferred embodiment, the steps include providing a computational processor and providing a radiation detector adapted to obtain the spectral intensity of the plasma. A database of spectral signatures for at least one substance is provided along with an output means communicatively coupled to the processor. The radiation detector, processor, and database are all communicatively coupled together wherein the spectral intensity is obtained by the radiation detector and measured values for the first and second wavelengths are passed onto the processor which calculates and compares the ratio to the database of spectral signatures and communicates the results to the output means. In another embodiment of the invention, the step of obtaining of the spectral intensity of the plasma may be time-gated to provide further discrimination of the sample""s plasma.
The present invention is not limited to minerals, but to any compound for which the identity is sought in rapid analytical fashion. For example, food products (i.e., ingestible matter) may be rapidly analyzed for compounds or elements of interest.